Field of the Invention
This application is concerned with improved polyurea-thickened greases containing alkali metal borate extreme-pressure agents prepared in situ.
Modern technology is currently supplying the general public and the process industries with machinery which is designed to operate under a wider range of temperatures and under greater loads than previously available. In addition, many of the newer machines are designed to operate at extremely high speeds. Many of these machines require certain specific lubricating properties which are not available in the conventional lubricants. Thus, modernization of high-speed and high-temperature equipment has strained the petroleum industry for the development of a second generation of lubricants capable of satisfying the requirements of the new machines. Recently, for example, there has been an increased demand for lubricants capable of performing well at temperatures above 300.degree. F in high-speed bearings and gears for periods in excess of 500 hours. In addition, with the further development of the high-speed sealed bearings, the lubricant must be able to endure for the life of the bearing.
There have been numerous grease compositions developed which satisfy most of the new, more stringent requirements. Many of these compositions, however, are entirely too expensive for commercialization or only meet some of the lubricating requirements and fail in others.
One type of grease composition which has excellent lubricating properties at the higher temperatures is comprised of a lubricating oil (natural or synthetic) containing a polyurea additive. This type of lubricant is disclosed in U.S. Pat. Nos. 3,242,210, 3,243,372, 3,346,497 and 3,401,027, all assigned to Chevron Research Company. The polyurea component imparts a significant high-temperature stability to the grease and in fact effects a mild anti-thixotropic property, i.e., increases in viscosity with increasing shear, to the lubricant. This property of the lubricant is advantageous to prevent the segregation or loss of grease from the moving parts of the machine. However, the polyurea component does not impart extreme-pressure properties to the lubricant and, accordingly, EP additives must be added in applications involving high contact pressures. A need therefore exists for a grease composition which can be used in high-temperature and high-speed applications that exhibits good stability over prolonged periods, that exhibits both extreme-pressure and antiwear properties, and that is relatively inexpensive to produce.
In the past a variety of agents have been employed as EP agents in greases. However, many of these compounds are corrosive to metal. Included among these are phosphorus, sulfur, and chlorine-containing additives such as the esters of acids of phosphorus, sulfurized olefins, sulfurized aromatic compounds, chlorinated hydrocarbons, etc. In addition, lead compounds have been employed as EP additives. Environmental concerns have, however, made it desirable to eliminate lead-containing additives from greases. Alkali metal borates, specifically sodium metaborate, have been incorporated in various greases as EP agents with varying degrees of success. When they were employed as thickening agents, however, with polyurea-thickened greases, while the EP characteristics of the greases were enhanced, it was discovered that workers using the greases suffered skin staining due to hydrolysis of the polyurea thickeners by the alkaline borates. Tests with rabbits show this grease also can cause skin irritation.
It is thus desirable that polyurea grease compositions be provided which possess good EP characteristics achieved without enhancement of metal corrosivity and without the skin staining problems associated with the use of the metaborate in polyurea-based greases.